Recently, a hard disk device is incorporated in a personal computer, a notebook personal computer, and a DVD (Digital Versatile Disc) recording apparatus in order to record data. Particularly, in the hard disk device used in an apparatus such as the notebook personal computer based on portability, a magnetic disk in which a magnetic layer is provided on a glass substrate is used, and magnetic recording information is recorded in or read from a magnetic layer using a magnetic head (DFH (Dynamic Flying Height) head) that is slightly floated on a surface of the magnetic disk surface. A disk-shaped glass substrate is suitably used as a substrate for the magnetic disk because the glass substrate is hardly plastically deformed compared with a metallic substrate, and is excellent in smoothness of the surfaces.
The magnetic recording density is being increased in order to correspond to a demand for an increase of a storage capacity in the hard disk device. For example, a magnetic recording information area is finely formed using a perpendicular magnetic recording system in which a magnetization direction of the magnetic layer is oriented toward a direction perpendicular to the substrate surface, which allows the storage capacity to be increased in one disk substrate. In order to correspond to the further increase of the storage capacity, a floating distance of the magnetic head from the magnetic recording surface is extremely shortened to form the fine magnetic recording information area. In the substrate of the magnetic disk, the magnetic layer is formed flat such that the magnetization direction of the magnetic layer is oriented toward the direction substantially perpendicular to the substrate surface. Therefore, the disk-shaped glass substrate for magnetic disk is formed such that the degree of surface irregularity of the glass substrate is decreased as much as possible.
The shortened floating distance of the magnetic head may easily cause a head crush trouble or a thermal asperity trouble. Because these troubles are generated by the micro irregularity or a particle on the magnetic disk surface, the disk-shaped glass substrate is formed such that the degree of surface irregularity at an end face in addition to that at the principal surfaces is also decreased as much as possible in addition to the principal surface.
For example, a disk-shaped sheet glass material used for a magnetic disk, namely glass blank, is manufactured by the following method: a glass gob made of molten glass is supplied onto a lower die that is a backing gob forming die; press forming is performed to the glass gob to prepare a glass blank using the lower die and an upper die that is a counter gob forming die; and a variety of machining is performed to obtain a glass substrate for magnetic disk (see PTL 1 below).
In the aforementioned method, after the glass gob made of the molten glass is supplied onto the lower die, the following steps are performed: a lower surface of a body for upper die and an upper surface of a body for lower die are abutted on together; a thin sheet glass forming space is formed outside a sliding surface between the upper die and the body for upper die and a sliding surface between the lower die and the body for lower die; the upper die is moved down to perform the press forming; and the upper die is moved up immediately after the press forming. Therefore, a sheet glass blank, which is a base of the glass substrate for magnetic disk, is formed. Then, the glass substrate for magnetic disk is obtained through a grinding process and a polishing process, etc.
In the grinding process, for example, grinding is performed using alumina loose abrasive grains. In the grinding process, a first grinding process and a second grinding process are performed using the loose abrasive grains having different particle sizes. A particle size of the loose abrasive grains used in the second grinding process is set smaller than that of the loose abrasive grains used in the first grinding process. Therefore, coarse grinding and fine grinding are performed in this order. Further, in the second grinding process, grinding using fixed abrasive grains is performed using a resin pad to which diamond abrasive grains are adhered.
The polishing process includes a first polishing process in which the loose abrasive grains such as a cerium oxide and a hard resin material polisher are used and a second polishing process in which colloidal silica and a soft resin material polisher are used. The particle size of the abrasive grain used in the first polishing process is smaller than that of the abrasive grain used in the second grinding process of the grinding process. The particle size of the abrasive grain used in the second polishing process is smaller than that of the abrasive grain used in the first polishing process.
As described above, in the surface processing of the glass substrate, the first grinding process, the second grinding process, the first polishing process, and the second polishing process are performed in this order, and the glass substrate is formed such that accuracy of surface quality such as surface roughness of the glass substrate is gradually enhanced.
A metallic die is known to manufacture a glass blank for information recording media whose outer end face and inner end face are chamfered. Protrusion which is wedge-shaped in section is provided at the both sides of such metallic die. Further, a manufacturing method is known, using such metallic die, to press form by holding molten glass from the both sides in the horizontal direction while the molten glass falls down (see PTL 2 below).